Broad bandwidth optical transmission systems have received a great deal of attention in recent years. Such systems require broad bandwidth optical amplifiers to achieve transmission of high capacity wavelength division multiplexed signals. A type of optical amplifier that is commonly employed incorporates rare-earth doped optical fibers as the gain medium. In particular, the rare-earth material that is often selected is erbium. Rare-earth doped optical fiber amplifiers are often referred to as discrete or lumped amplifiers, in contrast to so-called distributed amplifiers in which signal amplification occurs along the signal transmission path. An example of a distributed amplifier is a Raman amplifier.
Raman amplification is accomplished by introducing the signal and pump energies along the same optical fiber. The pump and signal may be copropagating or counterpropagating with respect to one another. A Raman amplifier uses stimulated Raman scattering, which occurs in silica fibers when an intense pump beam propagates through it. Stimulated Raman scattering is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e., optical phonons). That is, pump energy of a given wavelength amplifies a signal at a longer wavelength. The relationship between the pump energy and the Raman gain for a silica fiber is shown in FIG. 1. The particular wavelength of the pump energy that is used in this example is denoted by reference numeral 1. As shown, the gain spectrum 2 for this particular pump wavelength is shifted in wavelength with respect to the pump wavelength.
The Raman gain spectrum in a lossless transmission fiber, as graphically illustrated in FIG. 1, may be expressed as follows: EQU G(.lambda.)=.sub.e.sup.g.sup..sub.r .sup.(.lambda.)PL/A (1)
where P is the optical pump power, L/A is the effective length divided by the effective cross-sectional area of the pump beam inside the fiber, and g.sub.r (.lambda.) is the Raman gain coefficient, which is dependent on the material properties of the particular transmission fiber that is employed.
As FIG. 1 indicates, the bandwidth of the Raman amplifier is limited. For example, the bandwidth of the amplifier shown in FIG. 1 is only about 20 nm at 10 dB. Moreover, while much attention has been focused on expanding the bandwidth of rare-earth doped optical amplifiers, such a result has been difficult to achieve because of the properties of rare-earth doped fibers.
Accordingly, it would be desirable to provide an optical amplifier that has a gain profile with a wider bandwidth than is currently available from rare-earth doped fiber amplifiers and Raman amplifiers.